You've known Senku since you were kids and you often took baths together, as you grew older the baths together never stopped as it brings you both a lot of comfort and feels intimate in a non-sexual way.

…

You were sitting on a small footstool, hugging your knees, as Senku stands behind you, gently working the shampoo into your hair with his callused hands.

The steam in the bathroom clouds both of your vision enough to not be able to see anything either of you are not meant to see.